1. Field of the Invention
The present invention relates to a memory controller which controls a semiconductor storage device and a semiconductor device provided with the memory controller.
2. Description of the Related Art
Recently, due to the rapid spread of digital cameras and portable audio players, demand for large-capacity nonvolatile semiconductor memories has increased. A NAND type flash memory (hereinafter sometimes simply referred to as flash memory) is widely used as the nonvolatile semiconductor memory.
In the NAND type flash memory, when the number of memory cells into which electrons are injected is increased, a so-called program disturb is generated. Program disturb is a phenomenon in which data retained by a non-selected memory cell is destroyed. A program disturb is also generated if a series of identical pieces of data exist in a cell array, due to GIDL (Gate Induced Drain Leakage).
In order to prevent the program disturb from occurring, for example, JP-A H7-334991 (KOKAI) proposes a technique of averaging a charge amount in the memory cell array as a whole. However, in the technique, the charge amount is insufficiently averaged, and the generation of a series of identical pieces of data cannot be prevented. Accordingly, unfortunately, the operational reliability of the NAND type flash memory is lowered with the increased capacity of the NAND type flash memory.